A high ion-conducting, self-healing and nonflammable polymer electrolyte with dynamic imine bonds for dendrite-free lithium metal batteries

Highlights

• A self-healing polymer electrolyte based on dynamic imine bonds was developed.

• PBPE can completely self-heal within 1 h at room temperature.

• PBPE possesses high ionic conductivity of 4.79 × 10⁻³ S cm⁻¹ at 30 °C.

• PBPE can effectively suppress dendrite growth on Li metal anodes.

• LiFePO₄ cells assembled with PBPE exhibit excellent performance.

Abstract

High-performance polymer electrolytes with the capability to self-heal mechanical damage can effectively enhance reliability, safety and service life of the batteries. Herein, a novel polymer electrolyte (PBPE) with excellent self-healing capacity and high ionic conductivity was developed. PBPE with polymer networks cross-linked by highly reversible imine bonds was fabricated by the Schiff-base reaction between poly(ethylene glycol) diamine and benzene-1,3,5-tricarbaldehyde. PBPE can quickly repair the cut damage spontaneously within 1 h at room temperature and recover its mechanical strength and electrochemical properties. The healed PBPE exhibits almost the same ionic conductivities and mechanical properties as the original PBPE shows. PBPE possesses the highest ionic conductivity (4.79 × 10⁻³ S cm⁻¹ at 30 °C) among the self-healing polymer electrolytes. PBPE can promote the generation of LiF-rich SEI and effectively suppress dendrite growth on Li metal anodes. LiFePO₄ (LFP) cells assembled with PBPE exhibit excellent rate capability (discharge capacity of 118.2 mAh g⁻¹ at 5 C rate) and good cycling performance (capacity retention of 97.8% over 125 cycles). More importantly, the healed PBPE can completely recover its properties from damage in LFP cells and enable the cells to possess cycle performance identical to original PBPE.

Introduction

Lithium-ion batteries (LIBs) have been widely applied in electronic products and electric vehicles in recent years [1], [2]. Commercial LIBs usually consist of liquid electrolytes (LEs) and graphite anodes. Conventional LEs consisting of lithium salts and carbonate solvents are highly volatile and flammable [3], [4]. Combustions of LEs can lead to explosion, fire and thermal runaway [5], [6], [7]. There have been a number of reported accidents of fires and explosions of LIBs, such as explosions of Samsung note7 mobile phones and spontaneous combustions of Tesla cars [3], [5]. Hence, it’s vital to develop safer electrolytes to improve the safety of LIBs.

Solid electrolytes (SEs) are significantly safer than LEs, as they don't burn or explode [8]. SEs are able to restrain dendrite growth on Li metal anodes [9], [10]. Using SEs and Li metal anodes to assemble solid-state Li metal batteries (LMBs) is a promising technology to construct high-safety and high-energy-density batteries, owing to high theoretical capacity (3860 mAh g⁻¹) of Li metal anodes [11], [12], [13]. SEs can be classified into polymer electrolytes (PEs) and inorganic electrolytes (IEs) [14], [15], [16], [17], [18], [19]. IEs suffer from high interface resistance and brittleness [20], [21], [22]. Compared with IEs, PEs possess high flexibility, good interface compatibility, low density, high safety and good processability [23], [24], [25], [26], [27], [28]. However, ionic conductivities of all-solid-state PEs at room temperature (RT) are usually too low for application (10⁻⁷–10⁻⁵ S cm⁻¹) [29]. Decreasing crystallinities of polymeric matrixes, reducing glass transition temperatures of polymer matrixes and improving local segmental motion are the main strategies to accelerate lithium-ion conduction and improve ionic conductivities of PEs [30]. The corresponding methods include preparation of polymers with block, graft, star or cross-linked structures; incorporation of solvents, ionic liquids, oligomers or ceramic fillers in PEs; blending with different polymers; fabrication of organic/inorganic hybrid systems [31], [32], [33], [34], [35], [36], [37], [38]. However, mechanical strength of PEs will significantly decrease, when the crystallinity and glass transition temperature of the PEs are reduced. The flimsy PEs can be easily destroyed by external forces during the assembly and operation of the batteries, leading to blocking of lithium-ion conduction, short-circuit and short service life. The damaged PEs need the capability to self-heal the damage, which can effectively eliminate the safety hazard and significantly enhance the reliability of LMBs.

Self-healing is the capability to spontaneously repair mechanical damages and restore original functions [39]. Self-healing of polymers can be realized by introducing reversible covalent interactions or dynamic noncovalent interactions, including disulfide bonds, hydrogen bonds, host–guest interactions, metal–ligand interactions and ionic interactions between polymer chains [40], [41]. A number of self-healing PEs have been developed. A majority of the reported self-healable PEs are based on hydrogen bonds, which incorporate ureidopyrimidinone (UPy) units into the PEs to form self-complementary dimers via quadruple hydrogen bonds [39], [42], [43], [44], [45], [46], [47], [48], [49], [50]. For instance, Xue et al. [39] developed a PE (shPE) via copolymerization of UPy-containing monomer (UPyMA) and poly(ethylene glycol) methyl ethermethacrylate (PEGMA). Owing to reversible physical cross-linking by quadrupole hydrogen bonds of UPy, shPE can spontaneously heal the damage in 2 h at 30 °C and can be stretched to 2000% strain. The ionic conductivity of shPE at 30 °C was 2.1 × 10⁻⁵ S cm⁻¹. Compared with reversible non-covalent bonds, reversible dynamic covalent bonds possess higher bonding energy and stability, which endows PEs with better mechanical strength [51]. A few self-healing PEs based on reversible disulfide bonds [51], boronic ester bonds [52], [53] and ionic bonds [54], [55], [56] have been developed. Xue et al. [52] fabricated self-healing PEs (P2K-diGMPA) by copolymerization of poly(ethylene glycol) diacrylate and a crosslinker with boronic ester bonds. The transesterification reaction of boronic esters endows P2K-diGMPA with good self-healing performance. P2K-diGMPA can self-heal at 30 °C and the self-healed membrane can hang 20 g weight. At 30 °C, P2K-diGMPA possessed an ionic conductivity of 3.4 × 10⁻⁵ S cm⁻¹. So far, though these reported self-healing PEs possess self-healing capability, their ionic conductivities are relatively low at RT (~10⁻⁵ S cm⁻¹). The batteries assembled with these self-healing PEs can only charge/discharge at higher temperatures (60 °C) at low rates (0.1 C) [52]. Therefore, it’s crucial to develop self-healing PEs with high ionic conductivities.

Herein, we design and prepare an innovative PE (PBPE) with high ionic conductivity and excellent self-healing capability at RT. Imine bond, which is a highly reversible covalent bond and can carry out fast bond exchange without any side reactions [57], is used to achieve self-healing capability of PBPE. PBPE was prepared via the Schiff-base reaction between poly(ethylene glycol) diamine (NH₂-PEG-NH₂) and benzene-1,3,5-tricarbaldehyde (BTA), which resulted in polymer networks cross-linked by reversible imine bonds. In BTA, aromatic ring, as an electron acceptor, facilitates stabilization of zwitterion intermediate that has a negative charge on the imine nitrogen. Furthermore, imine exchange reactions (transamination) can quickly heal the cleaved imines [58]. Hence, such BTA-based imine bonds are highly reversible and stable. As a result, PBPE exhibits excellent self-healing capability and it can quickly repair the cut damage within 1 h. PBPE possesses the highest ionic conductivity (4.79 × 10⁻³ S cm⁻¹ at 30 °C) among the self-healing PEs. PBPE can effectively suppress dendrite growth on Li metal anodes. PBPE endows LiFePO₄ (LFP) cells with outstanding rate capability and good cycling performance. More importantly, PBPE can automatically heal the damage to restore the performances of LFP cells, significantly improving the reliability of the batteries.